The need to remain competitive in cost and performance in the production of semiconductor devices has caused a continuous increase in device density of integrated circuits. To accomplish higher integration and miniaturization in a semiconductor integrated circuit, miniaturization of a circuit pattern formed on a semiconductor wafer must also be accomplished.
Design rules define the space tolerance between devices or interconnect lines so as to ensure that the devices or lines do not interact with one another in any unwanted manner. One important layout design rule that tends to determine the overall size and density of the semiconductor device is the critical dimension (CD). A critical dimension of a circuit is defined as the smallest width of a line or the smallest space between two lines. Another critical design rule is minimum pitch, which is defined as the minimum width of a given feature plus the distance to the adjacent feature edge.
Photolithography is a standard technique utilized to manufacture semiconductor wafers by transferring geometric shapes and patterns on a mask to the surface of a semiconductor wafer. The basic photolithographic process includes projecting a patterned light source onto a layer of radiation-sensitive material, such as a photoresist layer, which is then followed by a development step.
To create finely detailed patterns with small critical dimensions and pitch requires projecting a clearly imaged light pattern. But the ability to project a clear image of a small feature onto the semiconductor wafer is limited by the wavelength of the light that is used, and the ability of a reduction lens system to capture enough diffraction orders from the illuminated mask. Current state-of-the-art photolithography tools use deep ultraviolet (DUV) light with wavelengths of 248 or 193 nm, which allow minimum feature sizes down to about 50 nm.
The minimum feature size that a projection system can print is given approximately by:CD=k1·λ/NA where CD is the minimum feature size or the critical dimension; k1 is a coefficient that encapsulates process-related factors, and typically equals 0.4 for production; λ is the wavelength of light used; and NA is the numerical aperture of the lens, as seen from the semiconductor wafer. According to this equation, minimum feature sizes can be decreased by decreasing the wavelength and/or by increasing the numerical aperture to achieve a tighter focused beam and a smaller spot size.
A photolithographic process utilizes an exposure tool to irradiate the layer of radiation-sensitive material on a wafer through a mask to transfer the pattern on the mask to the wafer. As the critical dimensions of the pattern layout approach the resolution limit of the lithography equipment, optical proximity effects (OPE) begin to influence the manner in which features on a mask transfer to the layer of radiation-sensitive material such that the mask and actual layout patterns begin to differ. Optical Proximity effects are known to result from optical diffraction in the projection system. The diffraction causes adjacent features to interact with one another in such a way as to produce pattern-dependent variations; the closer together features are, the more proximity effect is seen. Thus, the ability to locate line patterns close together encroaches on optical parameter limitations.
In accordance with the above description, new and improved methods for patterning semiconductor devices are therefore necessary, so as to accomplish the continued miniaturization of a circuit pattern formed on a semiconductor wafer. One non-optical approach is to narrow the line width of the radiation-sensitive material after the imaging, and the first developing are completed. Narrowing of line width is also known as “slimming” or “shrinking”, those terms being used herein synonymously.
As discussed above, patterning of a semiconductor wafer generally involves coating a surface of the wafer (substrate) with a thin film or layer of a radiation-sensitive material, such as a photoresist, and then exposing the layer of radiation-sensitive material to a pattern of radiation by projecting radiation from a radiation source through a mask. Thereafter, a developing process is performed to remove various regions of the radiation-sensitive material. The specific region being removed is dependent upon the tone of the material and the developing chemistry. As an example, in the case of a positive-tone photoresist, the irradiated regions may be removed using a first developing chemistry and the non-irradiated regions may be removed using a second developing chemistry. Conversely, in the case of a negative-tone photoresist, the non-irradiated regions may be removed using a third developing chemistry and the irradiated regions may be removed using a fourth developing chemistry. The removed regions of photoresist expose the underlying wafer surface in a pattern that is ready to be etched into the surface.
As an example for positive-tone pattern development, a typical lithographic patterning technique is shown in FIGS. 1A and 1B. As shown in FIG. 1A, a layer of radiation-sensitive material 102 is formed on a substrate 101. The layer of radiation-sensitive material 102 is exposed to electromagnetic (EM) radiation 107 through a mask 103. Reticle or mask 103 includes transparent regions 104 and opaque regions 108 that form a pattern, with a distance (or pitch) 109 being defined between opaque regions 108, as shown in FIG. 1A. The transparent regions 104 transmit EM radiation 107 to the layer of radiation-sensitive material 102, and the opaque regions 108 prevent EM radiation 107 from being transmitted to the layer radiation-sensitive material 102. As a result, the layer of radiation-sensitive material 102 has exposed regions 105 that are exposed to EM radiation 107 and unexposed regions 106 that are not exposed to EM radiation 107. As shown in FIG. 1A, opaque regions 108 are imaged onto the layer of radiation-sensitive material 102 to produce corresponding radiation-sensitive material features aligned with unexposed regions 106.
As shown in FIG. 1B, after removing exposed regions 105 of the layer of radiation-sensitive material 102 by a developing process, unexposed regions 106 remain on substrate 101 and form the pattern transferred from mask 103. The regions of radiation-sensitive material remaining after removal of the exposed regions 105 are referred to as radiation-sensitive material lines. As shown in FIGS. 1A and 1B, opaque regions 108 are imaged onto the layer of radiation-sensitive material 102 to produce corresponding radiation-sensitive material features (i.e., unexposed regions 106). As shown in FIGS. 1A and 1B, pitch 110 between unexposed regions 106 is determined by pitch 109 between opaque regions 108 of mask 103. In this example, the pitch 110 of the patterned feature is approximately twice the width of the critical dimension 111 of the radiation-sensitive material lines. Thus, the critical dimension 111 is determined by the distance between opaque regions of mask 103 and the development process. To further reduce the critical dimension 111 of the radiation-sensitive material lines requires additional processing, as discussed next.
One typical method for reducing radiation-sensitive material line width involves plasma-based etching of the unexposed region 106 of the radiation-sensitive material after a positive-tone development conducted at nominal temperature. Plasma-based etching suffers from various issues such as process stability and higher front end costs. Other slimming or shrinking methods include wet methods, such as performing a positive-tone development at elevated temperatures. But wet developing methods may suffer from anisotropic slimming caused by or exacerbated by variations in the photolithographic image, as will be discussed further below.
Additional details of the photolithographic image are provided in FIG. 2. A layer of radiation-sensitive material 202 is formed on a substrate 201. The layer of radiation-sensitive material 202 is exposed to EM radiation 207 through a mask 203. Mask 203 includes transparent regions 204 and opaque regions 208 that form a pattern, as shown in FIG. 2. A distance (or pitch) 209 between opaque regions 208 is shown in FIG. 2. The transparent regions 204 transmit EM radiation 207 to the layer of positive-tone radiation-sensitive material 202, and the opaque regions 208 prevent EM radiation 207 from being transmitted to the layer of radiation-sensitive material 202.
While it would be desirable to produce only two types of image patterns, i.e., exposed and unexposed, FIG. 2 shows three regions of radiation-sensitive material 202 having different levels of exposure to EM radiation 107. Exposed regions 205 and unexposed regions 206 are separated by a partially exposed region 214, wherein an exposure gradient extends across the width of partially exposed region 214. This exposure gradient may be affected by various factors, such as the radiation-sensitive material thickness, the depth of focus and proximity effect. Thus, this exposure variation or gradient induces anisotropic slimming, which may produce weak points in the radiation-sensitive material lines.
In view thereof, new methods of slimming radiation-sensitive material lines that overcome the problems of the prior art are needed.